A compound having a β-lactam ring in its molecule (hereinafter referred to as a “β-lactam compound”) is useful as an antibacterial agent with potent antibacterial activity. A wide variety of β-lactam compounds have been developed as antibacterial agents and various β-lactam compounds have been produced on an industrial scale.
Although such a β-lactam compound is characterized by having a β-lactam ring in its molecule, this β-lactam ring may be decomposed depending on the type of its substituent, the type of its condensed ring, and/or environmental conditions surrounding its solution (e.g., heat, the presence of water, the property thereof (acidic or alkaline)). For this reason, when β-lactam compounds are to be produced, as mild conditions as possible are selected for their production in order to prevent the compounds from becoming decomposed and causing side reactions during the production process.
For example, a β-lactam compound (4) useful as an antibacterial agent can be prepared by the reaction shown below.

However, when carrying out the above reaction to prepare β-lactam compounds on an industrial scale, there is a problem arising from a significant reduction in isolated yield during the step of isolating the target β-lactam compound (4) from the resulting reaction solution.